As is the case with purification,
absolute removal of moisture is not called for; all that is needed is to
extract so much that the gas shall never reach its saturation-point in
the inaccessible parts of the service during the coldest winter's night.
Any accessible length of main specially exposed to cold may be
safeguarded by itself; being given a steady fall to a certain point
(preferably in a frost-free situation), and there provided with a
collecting-box from which the deposited liquid can be removed
periodically with a pump or otherwise.
FILTRATION.--The gas issuing from the purifier or drier is very liable to
hold in suspension fine dust derived from the purifying or drying
material used. It is essential that thin dust should be abstracted before
the gas reaches the burners, otherwise it will choke the orifices and
prevent them functioning properly. Consequently the gas should pass
through a sufficient layer of filtering material after it has traversed
the purifying material (and drier if one is used). This filtering
material may be put either as a final layer in the purifier (or drier),
or in a separate vessel known as a filter. Among filtering materials in
common use may be named cotton-wool, fine canvas or gauze, felt and
asbestos-wool.
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